This invention relates to methods and apparatus for measuring the characteristics of microwave antennas.
It is frequently desirable to measure and compare the characteristics of a microwave antenna positioned in different physical positions. This has traditionally been done under "far-field" conditions where the distance between the test antenna or probe and the antenna under test is many times the diameter of the antenna under test. "Near-field" testing presents advantages, however, including the ability to perform such testing indoors in a small space.
Near-field testing requires determination of phase response, and measurement of phase response is also potentially desirable in other testing situations such as transmission of a microwave local oscillator signal to a harmonic mixer, which is sometimes done in far-field testing. In measuring the phase response of an antenna at different aspect angles or in performing near-field scanning of an antenna aperture, the antenna or an antenna probe must be moved mechanically. Such movement causes the cables to the antenna or probe to be flexed, thereby causing a change or error in the phase measurement because the path length through the cable is changed. This difference in length causes an error in phase measurements and makes it desirable to provide a phase stable reference or phase stable transmission path that is not affected by movement of the antenna or antenna probe.
In many far-field antenna ranges, the antenna cannot easily be located near the microwave receiver. Consequently, at high frequencies, it is advantageous to position a harmonic mixer at the output of the antenna in order to minimize radio frequency losses. In situations where the antenna moves, flexing occurs in the coaxial cable which transmits the local oscillator signal from the receiver to the harmonic mixer, thereby introducing phase instabilities and variations.